In recent years, mobile communication systems such as a Personal Handyphone System, which will be referred to as a “PHS” hereinafter, have been rapidly developed, and a PDMA (Path Division Multiple Access) method has been proposed for such mobile communication system. According to the PDMA method, one time slot of a single frequency is spatially divided so that mobile terminal devices of a plurality of users can be connected in a path division multiple manner to the radio base station system. In this PDMA method, a signal transmitted from the mobile terminal device of each user is separated and extracted by known adaptive array processing.
In the mobile communication system of the above PDMA type, a reception timing at which a signal transmitted from each mobile terminal device arrives at the radio base station (also referred to as a synchronization position) varies due to various factors such as changes in distance between a moving terminal device and the base station, and variations in propagation path characteristics of radio waves.
According to the mobile communication system of the PDMA type, the mobile terminal devices of a plurality of users may be connected in the path division multiple manner in the same time slot. In this state, the reception timing of the signal received from each mobile terminal device may vary due to the foregoing reasons so that the signal reception timing thereof may become close to that of another device. In some cases, the signal receiving order of the plurality of devices may be inverted.
If the reception timing becomes excessively close to anther, this increases a correlation value between the signals received from the plurality of mobile terminal devices, and lowers an accuracy of signal extraction for each user by the adaptive array processing. Therefore, the conversation characteristics for each user deteriorate.
In the PHS, a signal received from each mobile terminal device contains a reference signal section, which is formed of a known bit string common to respective users, in each frame. Therefore, if the signals are received from the mobile terminal devices of the plurality of users at the same reception timing, the reference signal sections of the received signals overlap with each other so that each user cannot be identified, and cross talk (so-called “SWAP”) occurs between the users.
Accordingly, the reception timings of the mobile terminal devices of the plurality of users, which are connected in the path division multiple manner in the same time slot, must be controlled to prevent such situations that the signal reception timing becomes close to that of another device, and that the signal receiving order of the plurality of devices is inverted.
For controlling the reception timing of signal received from the mobile terminal devices, it is effective to control the transmission timing for transmitting signals from the radio base station system to the mobile terminal devices.
By controlling the transmission timing for each user, the reception timing for each user can be controlled for the following reasons.
In the mobile communication system such as a PHS, the timing for transmitting and receiving signals between the radio base station system and the mobile terminal devices is determined according to the standards such that the mobile terminal device transmits a signal to the radio base station system after a predetermined time from reception of a signal from the radio base station system.
If the radio base station system shifts the signal transmission timing for each user from that of the other users, the timing of signal reception by the corresponding mobile terminal device is shifted from that of the others. Therefore, the timing for transmitting the signal from each mobile terminal device to the radio base station system shifts from that of the other mobile terminal devices.
As a result, the radio base station system receives the signal from each mobile terminal device at the timing shifted from the reception timing of the others.
As described above, by controlling the signal transmission timing of each mobile terminal device in the radio base station system, it is possible to control indirectly the timing for receiving the signal from each mobile terminal device by the radio base station system, and thereby the reception timing can be controlled to provide a longer reception timing interval with respect to the other.
In the mobile communication system of the PDMA type, however, if the number of users connecting in the multiple manner in each time slot increases, and thus if the path multiplicity increases, the transmission timing intervals in each slot inevitably decrease so that the receiving operations may be performed at reduced intervals or in an inverted sequence. In these cases, the conversation characteristics may deteriorate, or crosstalk may occur between user's devices.
Accordingly, an object of the invention is to provide a radio base station system and a transmission timing control method, which can maximize transmission timing intervals for a plurality of user's mobile terminal devices connecting in the path division multiple manner in the same time slot, and thereby can suppress deterioration of conversation characteristics and crosstalk between the users.